Description:
------------
The function strtoupper does not work with the danish letters ???. Here is an example:

<?php

echo strtoupper('abcdefghijklmnopqrstuvxyz???');

?>

Outputs 'ABCDEFGHIJKLMNOPQRSTUVXYZ???' when it should output 'ABCDEFGHIJKLMNOPQRSTUVXYZ???'.

Reproduce code:
---------------
<?php

echo strtoupper('abcdefghijklmnopqrstuvxyz???');

?>

Expected result:
----------------
ABCDEFGHIJKLMNOPQRSTUVXYZ???

Actual result:
--------------
ABCDEFGHIJKLMNOPQRSTUVXYZ???